The present applicant proposed by the patent publication 1 (Japanese Patent Laid-open publication 2010-94436) and the patent publication 2 (Japanese Patent Laid-open publication 2010-94441) a seat back frame structure of a seat for a vehicle with an oblique wire and a seat cushion frame structure of a seat for a vehicle with an oblique wire, respectively, in order to secure a necessary strength or rigidity upon the collision of the vehicle, while at the same time reducing the weight and the cost of the seat.
According to the patent publication 1, the seat back frame structure of a seat for a vehicle comprises a seat back frame structure, a lower end of which is connected to a rear end of a seat cushion frame structure in such a way that the seat back frame structure can be inclined relative to the seat cushion frame structure, said seat back frame structure includes a pair of side frames, each of which extends in a vertical direction of the vehicle, and an oblique wire on at least one of said pair of side frames, an upper end of which is fixed to a side portion of said at least one of the pair of side frames and a lower and of which is fixed to said seat back frame structure at a position forward of a rotational center of said seat back frame structure, said oblique wire includes a characteristic such that, in a case where a load toward a rear side of the vehicle is loaded on said seat back frame structure, it has a tension force exerted on said seat back frame structure so as to alleviate a moment acting on said seat back frame structure caused by said load, while it does not resist a compressive force from outside thereof.
According to the seat back frame structure of a seat for a vehicle, a sectional shape of at least one of a pair of side frames can be determined so as to set a section modulus of at least one of the pair of side frames in such a way that at least one of the pair of side frames can withstand a bending moment caused by an impact load loaded toward the front side of the vehicle, and in a case where an impact load larger than the impact load loaded toward the front side of the vehicle is loaded toward the rear side of the vehicle, it is possible to cause an oblique wire to share a difference between said impact load toward the front side of the vehicle and that toward the rear side of the vehicle by positioning the oblique wire so as to cause a tension force from the oblique wire to be exerted on the seat back frame structure in such a way that said bending moment acting on the seat back frame structure based on said impact load toward the rear side of the vehicle can be alleviated, and as a result, since it becomes unnecessary to set a sectional shape of the pair of side frames so as to withstand the impact load toward the rear side of the vehicle, a necessary strength or rigidity of the pair of side frames can be secured, while at the same time the weight of the pair of side frames can be decreased by means of the oblique wire.
On the other hand, according to the patent publication 2, the seat cushion frame structure of a seat for a vehicle comprises a seat cushion frame structure, a rear end of which is connected to a lower end of a seat back frame structure in such a way that the seat back frame structure can be inclined relative to the seat cushion frame structure, said seat cushion frame structure includes a pair of side frames, each of which extends in a longitudinal direction of the vehicle, and an oblique wire on at least one of said pair of side frames, an upper end of which is fixed to a side portion of said at least one of pair of side frames and a lower end of which is fixed to said seat cushion frame structure at a position forward of said upper end, said oblique wire includes a characteristic such that, in a case where a load toward a rear side of the vehicle is loaded on said seat cushion frame structure, it has a tension force exerted on said seat cushion frame structure so as to alleviate a moment acting on said seat cushion frame structure caused by said load, while it does not resist a compressive force from outside thereof.
According to the seat cushion frame structure of a seat for a vehicle, a sectional shape of at least one of a pair of side frames can be determined so as to set a section modulus of at least one of the pair of side frames in such a way that at least one of the pair of side frames can withstand a bending moment caused by the impact load loaded toward the front side of the vehicle, and in a case where an impact load is larger than the impact load loaded toward the front side of the vehicle is loaded toward the rear side of the vehicle, when such an impact load is transmitted to the seat cushion frame structure through a connection portion between the seat cushion and the seat back, it is possible to cause an oblique wire to share a difference between said impact load toward the front side of the vehicle and that toward the rear side of the vehicle by positioning the oblique wire so as to cause a tension force from the oblique wire to be exerted on the seat cushion frame structure in such a way that said bending moment acting on the seat cushion frame structure based on said impact load toward the rear side of the vehicle can be alleviated, and as a result, since it becomes unnecessary to set a sectional shape of the pair of side frames so as to withstand the impact load toward the rear side of the vehicle, a necessary strength or rigidity of the pair of side frames can be secured, while at the same time the weight of the pair of side frames can be decreased by means of the oblique wire.
However, the present applicant found out the following technical problems caused by the adoption of such an oblique wire.
Firstly, in such an oblique wire, its apparent tension rigidity can be decreased upon the collision of the vehicle, and the cost for the material and manufacturing of it is rather high.
More specifically, the conventional oblique wire includes an expensive wire formed by bundling a plurality of helical stranded lines and a separate ring portion fixed on each end of the wire by the caulking, so that it is fixed on a seat frame structure through the respective ring portions. In such a case, a slippage between the adjacent stranded lines can be caused when a tension force is generated on the oblique wire due to the fact that the stranded lines are caused to be straightened under the wire structure in which the plurality of stranded lines are bundled, while at the same time, a slippage can be also caused at the caulking portions of the ring portions fixed on the upper and lower ends of the oblique wire during the caulking process, whereby an apparent tension rigidity of the oblique wire is decreased.
Secondly, even if such an oblique wire is adopted with respect to the seat back frame structure, as compared with a case where it is adopted with respect to the seat cushion frame structure, it is technically difficult for the oblique wire to effectively bear the impact load.
More specifically, as shown in FIG. 23, the larger an inclination angle α of the oblique wire becomes, the bigger a rate of a component resisting the impact load becomes, so that the provision of the oblique wire is effective. However, as compared with the seat cushion frame structure, in the seat back frame structure, it is difficult to increase the inclination angle α due to the seat frame structure, so that the above effectiveness is reduced.
Further, as shown in FIG. 23, in the back frame structure (FIG. 23(A)), the oblique wire W is provided on the side portion of the side frame of the seat back frame structure. In this case, it is evident that the height H of the upper end P2 of the oblique wire at the rear side of the vehicle is bigger than the horizontal protruding distance L of the lower end P1 of the oblique wire W at the front side of the vehicle from the opposed side frames, so that the inclination angle α inevitably becomes smaller than 45 degrees. On the other hand, in the cushion frame structure (FIG. 23(B)), the oblique wire W is also provided on the side portion of the side frame of the seat cushion frame structure. In this case, it is evident that the horizontal protruding distance L of the lower end P1 of the oblique wire W at the front side of the vehicle is bigger than the vertical protruding distance L of the upper end P2 of the oblique wire W at the rear side of the vehicle from the opposed side frames, so that the inclination angle α inevitably becomes larger than 45 degrees, contrary to the case of the seat back frame structure, and thus, it is difficult to make the horizontal protruding distance L long in the seat back frame structure, mainly because the aesthetic appearance of the seat back can deteriorate if the horizontal protruding distance L is made long.